The present invention relates to temperature controlled soldering irons, and particularly, to improvements in electrically controlled soldering irons having a soldering tip including both heating and temperature sensing apparatus.
Soldering irons are widely used in the electronics industry to heat electrical components and solder thereby melting the solder and forming both electrical and mechanical connections for the electrical components. Many types of components are connected in this manner and many different types of solder are used to make connections. The various components have varying sensitivities to the temperatures applied to their connecting leads and the various solders have different optimum melting points. Accordingly, it is desirable to have soldering irons with which the tip temperature can be regulated to best suit the solder and components being used.
One type of soldering iron includes a heating element having a resistance which varies in accordance with the heat generated by that heating element. As the heating element gets hot, the resistance increases thereby reducing the current flow through the heating element and thus, reducing the temperature generated by that heating element. The temperature of such heating elements can be controlled by manually varying the voltage applied to the heating element. Such control however, is only an approximation.
For more exact temperature regulation of the tip, soldering irons are known which include a substantially constant resistance heating element and a separate temperature sensing element in the soldering iron tip. A control apparatus continually surveys the tip temperature, as indicated by the temperature sensing element and regulates the voltage applied to the heater to maintain a constant preset temperature. One soldering iron having a heater and a separate temperature sensing element in the tip is disclosed in U.S. Pat. No. 3,699,306 to Finch.
Recent advances in semiconductors, particularly advances in integrated circuitry, have created electronic components which are more sensitive to higher temperatures than components were in the past. Equally important, the new semiconductor circuitry is extremely sensitive to stray voltages applied to the conductive leads thereof. The increased component sensitivity to applied voltages has been recognized by the United States Government which has promulgated a standard MIL-STD-2000 for soldering irons. MIL-STD-2000 specifies that the soldering iron tip to ground leakage voltage shall be less than 2 millivolts. This standard is difficult, if not impossible, to achieve with soldering irons of the type disclosed in the aforementioned Finch patent.
A need exists for soldering equipment having the accurate temperature controllability provided by separate heating and heat sensing elements in the soldering iron tip, while at the same time, limiting the leakage voltage at the tip below values required by government standards and modern semiconductor equipment.